= Mixing-length closure
{title2=$\nu_t\sim\ell^2|\partial_zU|$}
A mixing-length closure uses a prescribed <mixing length> $\ell$ to approximate turbulent transport. For mean shear $\partial_zU$, a standard model gives <eddy viscosity> $\nu_t=\ell^2|\partial_zU|$ and down-gradient <Reynolds stress>. Scalar-diffusion versions replace momentum transport by an appropriate scalar flux. The choice of length and correlations is a modelling assumption rather than an exact closure of the <Navier-Stokes equation>.
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